Cell-surface heparan sulfate proteoglycans are essential components of the unconventional export machinery of FGF-2

Abstract

FGF-2 is an unconventionally secreted lectin that transmits proangiogenic signals through a ternary complex with high-affinity FGF receptors and heparan sulfate proteoglycans (HSPGs). Although FGF-2 signal transduction is understood in great detail, its mechanism of release from cells, which is independent of the classical secretory pathway, remains elusive. To test the hypothesis that FGF-2 secretion is linked to its cell-surface ligands, we studied FGF-2 release using mutants defective for HSPG binding and cells with impaired HSPG biosynthesis. Here, we report that a functional interaction between FGF-2 and HSPGs is required for net export of FGF-2 from mammalian cells. FGF-2 release requires extracellular, membrane-proximal HSPGs. We propose that extracellular HSPGs form a molecular trap that drives FGF-2 translocation across the plasma membrane.

Phosphatidylinositol 4,5-Bisphosphate (PI(4,5)P2)-dependent Oligomerization of Fibroblast Growth Factor 2 (FGF2) Triggers the Formation of a Lipidic Membrane Pore Implicated in Unconventional Secretion

“…Because of this, a toroidal pore is considered to have structural flexibility. In case of FGF2, this is consistent with previous observations demonstrating that various unrelated domains such as GFP can be fused to FGF2 without a complete loss of membrane translocation activity (24,26,51).…”

“…In conclusion, this study provides a biochemical and biophysical explanation for observations in living cells where FGF2 secretion was shown to occur by direct translocation across the plasma membrane (5,24) as well as to depend on PI(4,5)P 2 -mediated membrane recruitment (22), tyrosine phosphorylation (29), and maintenance of a folded conformation of FGF2 (26,27). They are likely to also be relevant for other type I (Fig.…”

“…7). At this stage, heparan sulfate proteoglycan-dependent translocation of FGF2 into the extracellular space can occur (23,24) resulting in the disassembly of the membrane pore. Because FGF2 has about a hundredfold higher affinity to heparan sulfate proteoglycans compared with PI(4,5)P 2 (22) and binding of FGF2 to either PI(4,5)P 2 or heparan sulfates is mutually exclusive, we propose that heparan sulfate proteoglycans liberate FGF2 from the membrane-associated state resulting in its exposure on cell surfaces.…”

“…FGF2 secretion from living cells occurs by direct protein translocation across plasma membranes (5,24). This process depends on PI(4,5)P 2 -mediated recruitment at the inner leaflet of the plasma membrane (22), tyrosine phosphorylation (29), and maintenance of folding during FGF2 membrane translocation (26,27).…”

“…At the outer leaflet, membrane-proximal heparan sulfate proteoglycans are involved in a late step of FGF2 secretion resulting in its exposure on cell surfaces (23,24). These sequential interactions are essential, as experimental conditions that prevent the ability of FGF2 to interact with either PI(4,5)P 2 or heparan sulfates cause inhibition of FGF2 secretion (22,24,25).…”